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Digital Automatic Level Control for Analog-to-Digital Conversion

The severity of overloads in an analog-to-digital converter circuit (A/D) as the analog input signal exceeds the dynamic range of the A/D has long been a target for any designer to solve. Not only the analog signal spectrum becomes infested with fast-rising clipping distortion but the harmonics generated by clipping the high frequencies can fold back and aggrevate the aliasing problem upon digital-to-analog (D/A) conversion. One obvious solution is to place an automatic gain control (AGC) circuit before the A/D, but analog methods generally have failed since the AGC must be transparent, i.e. no degradation, to the A/D; and be threshold, i.e. inactive until the analog signal approaches overload level; not to mention the side effects of breathing or signal pumping. With the advent of high-performance digitally-controlled attenuators (DCA's), a digital signal processor (DSP) can be utilized to provide precise threshold, instantaneous attack and many programmable features that suffers no drawbacks or side effects of the analog design. Further enhancement techniques can also provide more sophistication such as adaptive control process.